Conventionally, various apparatuses such as a vehicle are provided with a ball joint. For example, in a suspension of an automobile, a ball joint is fitted to a knuckle provided in a vehicle body. A dust cover is used on the ball joint in order to prevent water, dust, and the like from entering a joint section and to prevent grease from flowing out from the joint section. A ball joint according to a conventional example will now be described with reference to FIG. 8. FIG. 8 is a schematic sectional view of a ball joint according to a conventional example.
A ball joint 600 includes a ball stud 610 having a spherical section 612 at one end of a shaft section 611, a socket 620 which rotatably and swingably supports the ball stud 610, and a dust cover 630 which suppresses dust from entering the socket 620. In addition, the dust cover 630 has a seal main body which is made of an elastic body and which integrally includes a deformable film-like body section 631, a fixed section 632 which is provided at one end of the body section 631 and which is fixed to the socket 620, and an inner circumferential seal section 634 and a dust seal section 635 which are provided at another end of the body section 631. Moreover, the inner circumferential seal section 634 slides with respect to the shaft section 611 of the ball stud 610 and the dust seal section 635 slides with respect to the knuckle (not shown) described earlier.
The ball joint 600 configured as described above is shipped to vehicle manufacturers and the like in a state where the ball joint 600 is unitized by the ball stud 610, the socket 620, and the dust cover 630. Subsequently, at a shipping destination, the ball joint 600 is fitted to a knuckle (not shown) provided in a vehicle body. Specifically, the ball joint 600 is fitted to the knuckle by inserting the shaft section 611 into an insertion hole provided on the knuckle and fastening the shaft section 611 by a nut. In the process of the shaft section 611 being inserted into the insertion hole, the dust cover 630 is pushed by the knuckle in a direction of an arrow P in FIG. 8 and the body section 631 of the dust cover 630 bulges outward.
If the ball joint 600 is fitted to the knuckle in a state where the body section 631 bulges outward, the body section 631 deforms effortlessly when the ball stud 610 swings or rotates. However, there may be cases where a part of the body section 631 deforms so as to buckle inward when the ball joint 600 is fitted to the knuckle. A dotted line 631X in FIG. 8 represents an example of a state where a part of the body section 631 has deformed so as to buckle inward. When the ball stud 610 swings or rotates in a state where the body section 631 is deformed in this manner, there is a risk that stress may concentrate on a part of the body section 631 to cause damage.
When the dust cover 630 is pushed in the direction of the arrow P from a state where the body section 631 is at its natural length, a buckling deformation as described above is unlikely to occur. However, there may be cases where the dust cover 630 is stretched longer than its natural length at the time of shipping or the like. FIG. 8 shows a state where the dust cover 630 is stretched longer than its natural length by H. When the dust cover 630 is pushed in the direction of the arrow P from a state where the dust cover 630 is stretched longer than its natural length in this manner, a buckling deformation as described above is likely to occur. In recent years, progress has been made in downsizing ball joints and diameters of the socket 620 and the dust cover 630 tend to be smaller. Accordingly, problems such as that described above are becoming more apparent.